The invention relates to an antilocking braking control system for a road vehicle having a hydraulic multiple circuit brake system with at least one closed brake circuit which is connected to an output pressure space of a brake booster in which a static pressure, proportional to the force of actuation, is generated. This pressure is connected to a primary chamber of a pressure modulator provided to control pressure reduction, pressure build-up, as well as pressure maintenance phases of the antilocking control system for the at least one wheel brake of the static brake circuit via an inlet valve, which is mechanically drivable into its open position. The wheel brake can be connected to, or alternatively closed off from, the primary chamber of the pressure modulator via an electrically controllable brake pressure control valve, formed as a 2/2-way solenoid valve.
The inlet valve, which is mechanically drivable into an open position, is located in a primary chamber of a pressure modulator for controlling pressure reduction and pressure build-up, as well as pressure maintenance phases of the antilocking control system, to at least one wheel brake of the static brake circuit. The wheel brake is connectable to, and alternatively closed off from, the primary chamber of the pressure modulator.
The antilocking system has a hydraulic multiple circuit wheel brake system, comprising at least one closed static brake circuit which is connected to at least one wheel brake from an output pressure space of a brake booster in which an output pressure proportional to an actuation force is generated.
The connection includes an inlet valve means for connecting the brake booster to a primary chamber of a pressure modulator means having a pressure output line leading to at least one wheel brake via at least one electrical 2/2-way solenoid controllable brake pressure control valve means for controlling pressure build-up, pressure reduction and pressure maintenance phases respectively of said antilocking control system.
The one wheel brake is connected to or closed from the output of the primary chamber by the brake pressure valve means. The pressure modulator means has a hydraulically driven pump with a drive pressure space connected to the pressure modulator means to displace the piston pump. An electrically controllable ABS function control valve means connecting the drive pressure space alternatively to a pressure output of a hydraulic auxiliary pressure source and to its pressureless reservoir.
The piston pump separates the primary chamber from the drive pressure space. The piston pump is displaceable by the pressure in the drive pressure space when the inlet valve is open, with the piston pump displacement being opposed by a powerful restoring spring and against pressure prevailing in the primary chamber, and being displaceable to an end position, associated with a minimum volume of the primary chamber when the drive pressure space is connected to the auxiliary pressure source by the ABS control valve, and to another end position associated with a maximum volume of the primary chamber by action of the restoring spring, when said ABS control valve connects the drive pressure space to the reservoir of the auxiliary pressure source.
The maximum volume expansion or minimum volume reduction of the primary chamber corresponds to between 25% and 50% of the volume of brake fluid quantity which can be expelled by an actuation of said brake booster with the maximum force actuation which can be applied into said brake circuit.
An electronic ABS control unit generates control signals required for the appropriate control of the ABS control valve and of the brake pressure control valve in response to vehicle wheel speed proportional output signals from vehicle wheel speed sensors associated with the vehicle wheels.
An antilocking system of this type is the subject of Applicants' German Unpublished patent application P 36 37 781.3-21, which corresponds to U.S. patent application Ser. No. 07/116,867, filed Nov. 5, 1987, and now pending in Patent Examining Group 314.
In the antilocking control system described therein, the pressure modulator associated with the closed brake circuit, for example the front axle brake circuit, is of the type of a hydraulically driven pump having a feed pressure space (the primary chamber) which is movably defined and separated from a secondary chamber. (the drive pressure space) by a modulator piston . The drive pressure space is connectable by means of an ABS function control valve to the high pressure output of an auxiliary pressure source, whereby the modulator piston is pushed into an end position, associated with a minimum volume of the primary chamber against the action of a powerful restoring spring. In this end position of the piston, an inlet valve, designed as a ball-seat valve, is held in its open position by a tappet supported on one side by a valve ball, and on the other side by the piston, as a result of which the output pressure space of the brake booster which is connected with inlet valve, communicates with the primary chamber.
The two front wheel brakes are each connected to an output of the primary chamber via a brake pressure control valve designed as a 2/2-way solenoid valve. In the normal operating state of the brake system, i.e., when the brake system is not subjected to antilocking control, the high output pressure of the auxiliary pressure source is connected to the drive pressure space of the pressure modulator, and the piston of the latter is thereby held in the end position associated with a minimum volume of the primary chamber.
The piston and the auxiliary pressure source are so dimensioned that the piston remains in the above-mentioned end position, even if a braking action is performed with the maximum brake pressure. This maximum pressure is connected to the wheel brakes via the open inlet valve, the primary chamber and brake control valves when in their basic positions. If a locking tendency occurs at one of the braked wheels, which requires a pressure reduction in the wheel brake, then the brake pressure control valve of the other wheel brake is driven to its blocking position, and the ABS control valve is driven to an excited position, in which the control pressure previously connected to the drive pressure space of the pressure modulator can be released towards a pressureless reservoir of the auxiliary pressure source. As a result, the piston is subjected to a displacement in the sense of a volume enlargement of the primary chamber, under the influence of the restoring spring and the brake pressure in the primary chamber. The inlet valve reaches its blocking position after a small initial displacement of the piston in this displacement direction, and a pressure reduction at the brake to be controlled is achieved as the volume of the primary chamber is further enlarged.
The pressure modulator is s dimensioned that the maximum pressure reduction stroke of the modulator piston is sufficient for this, even if previously almost all of the maximum possible brake pressure was coupled into the wheel brakes and has to be reduced to a large degree by the control system. A pressure reduction of this kind, within one piston stroke, however, is no longer possible when the control system is effective on both wheel brakes.
For example, when a detected output signal of a position indicator, provided for the recording the piston position and characteristic of the piston position, no longer changes while the control system still continues to call for a pressure reduction at the wheel brakes, the brake pressure control valves of both wheel brakes are driven to their locking position.
The drive pressure space of the pressure modulator is again connected to the high output pressure of the auxiliary pressure source, as a result of which the modulator piston is then subjected to a displacement in the sense of a reduction of the volume of its primary chamber. The inlet valve has a construction corresponding to a non-return valve which opens as a result of the pressure building up in the primary chamber and brake fluid being expelled back out of the primary chamber into the tandem master cylinder. The pressure modulator functions in this phase like the return feed pump of known antilocking control systems.
After completing such a return feed stroke of the pressure modulator, the pressure reduction mode can be continued again by renewed displacement of the piston in the sense of an enlargement of the volume of the primary chamber. In order that the brake system, while operating without the antilocking control functions, remains functional even when the auxiliary pressure source has failed and the piston of the pressure modulator withdraws to its end position associated with a maximum volume of the primary chamber, a bypass valve designed as a ball-seat valve is provided which releases a second pressure medium flow path leading from the brake booster to the primary chamber of the pressure modulator as soon as the piston has reached a minimum distance, corresponding to the greatest part of its maximum stroke.
The valve ball, which up until then was pushed into its blocking position by a spring-loaded closing element, is taken along with the piston from the point where the latter reaches a minimum stroke, so that the ball can separate from the valve seat.
Irrespective of numerous advantageous functional features of this antilocking control system in comparison to antilocking control systems of similar construction, an unfavorable feature is evident in the case where the auxiliary pressure source, although it has not failed completely, supplies only a reduced output pressure. There, the modulator piston can no longer be pushed into its end position, associated with a minimum volume of the primary chamber, by subjecting its drive pressure space to high pressure. This end position is also utilized for the non-controlled braking mode, so that the piston withdraws slightly under the influence of the brake pressure connected to the primary chamber, as a result of which the inlet control valve can reach its blocking position even during the normal braking mode, and this leads to at least a restriction of the utilizable brake pressure, which of course is unacceptable.
Even more unfavorable is of course the case when the output pressure of the auxiliary pressure source is no longer sufficient to displace the piston, against the bias restoring force of the powerful restoring spring, at least close enough to its end position, associated with a minimum volume of the primary chamber. Such a displacement is necessary for the valve ball of the inlet valve to be lifted off its valve seat during at least part of its normal opening stroke, and for the bypass valve to remain held in its blocking position by the still partially possible displacement of the modulator piston. If this displacement does not occur, it is no longer possible to brake at all with the brake circuit connected to the pressure modulator. The relatively complicated mechanical construction of the pressure modulator, including the inlet and the bypass valves, can also be seen as a disadvantage, albeit a less grave one.
The object of the invention is therefore to improve an antilocking control system of the type aforementioned that is less susceptible to malfunctioning and is of a simple construction, and in particular, one that ensures that the brake system remains functional even if the auxiliary pressure source fails. This insurance of function is obtained by utilization of a bypass flow path as will be explained subsequently.
This object is achieved according to the invention by having the inlet valve means operating as a change over solenoid valve which in normal non-ABS brake mode, occupies a basic position connecting the primary chamber of the pressure modulator to the pressure output of the brake booster. The inlet valve means is controlled by output signals of the electronic control unit of the ABS to an excited "I" position closing off of the primary chamber of the pressure modulator from the pressure output of the brake booster, during pressure reduction as well as pressure build-up phases of the antilocking control system. The inlet valve means is switched back to its basic "O" position when the pressure modulator functions in the return feed mode at the same time as the brake pressure control valve means of the wheel brakes connected to the pressure modulator are switched back to excited "I" positions.
As a result of the design of the inlet control valve as a solenoid valve, this inlet valve can be driven independently of the current position of the modulator piston into the respective function position appropriate to the situation. This ensures that braking is possible in each possible position of the piston. The additional technical expenditure entailed by the solenoid valve is offset by a considerable simplification of the construction of the pressure modulator, since both a mechanical inlet valve and a mechanical bypass valve along with the actuation or closing elements necessary for their control, are dispensed with. As a result, the pressure modulator can be manufactured considerably cheaper and also with smaller dimensions.
As a result of having an antilocking system, wherein a position indicator is provided which generates electrical output signals characteristics of the position of the pressure modulator piston pump, which are connected to the electronic ABS control as further information inputs, and when the inlet control valve means is switched back to its basic position during an antilocking control cycle, comprising several pressure reduction, pressure maintenance, and pressure build-up phases, when the ABS control valve has been switched to its basic "O" position and the at least one brake pressure control valve means is in its excited "I" position, and wherein the output signals of said position indicator indicates that the piston pump is moving to its end position associated with a minimum volume of the primary chamber, a preferred embodiment of the antilocking system is possible which provides simple logical combinations, to control the solenoid valve in a sequence and duration appropriate for control.
It is achieved after having the inlet valve control valve driven to its excited "I" position in the course of an antilocking control phase, that the inlet control valve is switched back into its basic "O" position even if in the course of control, vehicle deceleration, detectable as a result of the change of output signals of the wheel speed sensors, lessens by more than a given threshold value, that the pressure in the primary chamber is matched as quickly as possible to the control pressure generated by the driver through the brake booster. This is true regarding a building-up of brake pressure, as well as a reduction of brake pressure.
When the wheel brake subjectable to the antilocking control system is connected via a one-way valve to a bypass line leading to the pressure output of the brake booster, it is advantageous to have the bypass line connected to the wheel brakes via a one-way valve which, when the driver throttles back the brake pedal during a control phase, brake pressure can be reduced rapidly.
As a result of the fact that the ABS control valve has a flow path, effecting the connection of the pressure output of the auxiliary pressure source to the driver pressure space of the pressure modulator when the ABS control valve is in the basic "O" position, via a throttle which restricts the pressure medium flow rate to the drive pressure space in the pressure modulator to a given value, this enables the change rates of the brake pressure in pressure build-up and pressure reduction phases of the antilocking control system to be placed in an optimum relation to each other.
A further ABS controlled function control is provided between the ABS function control valve means and a pressure reservoir of an auxiliary pressure source chargeable by an accumulator-type charge pump. The pressure reservoir is maintained at a specific output pressure level by opening a pressure-limiting valve connected between the pressure output of the pump and a return line leading to a reservoir of the pump, while the further function control valve means is connected, via a one-way valve, to the ABS function control valve means when the high pressure output of the pump is closed from the return line and the further function valve is in its basic position. The further function valve, in its excited position, closes off the pressure reservoir from the pressure output of the auxiliary pressure source while the pressure output of the pump is connected to the return line leading to a pressureless reservoir of the auxiliary pressure source.
With this arrangement pressure maintenance phases of the antilocking control system and at the same time the charging of a pressure reservoir are provided within the framework of the auxiliary pressure source and can be controlled in a simple manner. It is also possible with the aid of this further function valve to operate the accumulator-type charge pump sparingly by utilizing a pressure limiting valve, the opening pressure of which determines the reservoir pressure level.
The pressure modulator has a step cylinder having at least two bore steps of different diameter which merge into one another via housing step; and a correspondingly stepped modulator piston pump with a larger piston step forming a movable delimitation of the drive pressure space and a smaller piston step a movable delimination of the primary chamber of the pressure modulator. The piston is sealed against the respective stepped bore and has a central tapering section with a conical outer surface on which an actuating pin of a piston indicator is radially supported. The piston position indicator comprises a travel/voltage converter, by means of which radial displacements of said actuating pin associated with axial movements of said piston pump are converted into voltage signals which can be processed by the electronic control unit of the ABS. As a result thereof a simple embodiment of the modulator piston and position indicator suitable for monitoring its position are provided to permit an exact establishment of the current position of the piston as well as its speed of travel.
An additional feature of the invention is the restoring spring, which biases the modulator piston into its end position associated with a maximum volume of the primary chamber. It is designed as a helical pressure spring which is accommodated in a central blind bore of the modulator piston in a section of its length, approximately corresponding to an axial extension of the smaller piston step and the central conical section of the piston. As a result, the restoring spring still develops a desirable amount of force even in the end position of its piston, i.e., the end associated with a maximum volume of the primary chamber of the pressure modulator.
When the modulator piston is supported in the housing of the pressure modulator, an additional restoring spring, the restoring force of which is of a maximum at the position associated with a minimum volume of the primary chamber, also biases the piston as it is displaced increasingly away from this minimum volume position. The force of this additional restoring force reduces to a zero value at approximately 1/4 to 1/8 of the total stroke of the piston. A considerable restoring force is thus available in the particular one stroke region of the piston directly adjacent to the end position associated with a minimum volume of the primary chamber. The two restoring springs have a suitable dimensioning wherein the sum of the maximum values of the restoring forces of the biasing springs and the force resulting from a maximum output pressure of the brake booster acting upon the piston in the primary chamber, all operate in the sense of an enlargement of the primary chamber volume, and are of a value which is at least less than, or approximately equal to, the force in the driven pressure space which displaces the piston into its end position associated with a minimum volume of the primary chamber when the modulator piston is subjected to the output pressure of the auxiliary pressure source. Such a control system allows for counter-phase control, i.e., the brake pressure can be built up in one wheel brake, while brake pressure is reduced at the other wheel brake.
By having a 2/2-way solenoid bypass valve means connected between a pressure output of the brake booster and the connection, leading from a pressure output of the primary chamber of the pressure modulator to the at least one wheel brake, and wherein the bypass valve means and the inlet valve means are controlled together to move from a basic "O" position into an excited blocking "I" position, the reliability of the antilocking system and the brake system as a whole can be improved.
It is advantageous to have the modulator piston include an electronic pressure sensor to provide an output indicative of the pressure in the primary chamber of the modulator piston to the ABS electronic control.
The pressure sensor can comprise a sensor element fixed between two separated parts of the piston and which is subject to a compression or deformation due to at least one of the piston parts being responsive to the pressure in the primary chamber of the pressure modulator. The sensor could be any of a compression-sensitive or expansion-sensitive electrical resistor or piezoelectric element.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.